1. Field of the Invention:
The present invention relates to a solid electrolyte fuel cell that converts the free energy of a solid electrolyte to electrical energy by an electrochemical reaction and, in particular, to the reliability, operating characteristics and thermal efficiency of such a cell.
2. Description of the Related Art:
Conventional power generators which employ heat engines such as gas turbines and diesel engines are subject to Carnot cycle efficiency limits. In contrast, the efficiency of fuel cells is dependent on the relationship between the change in the free energy of the underlying electrochemical reaction and the enthalpy of that reaction, and hence is expected to be higher than the efficiency of conventional power generators. Because of this advantage, the use of fuel cells as power generators will contribute to efficient utilization of resources and to a relative decrease in the level of CO.sub.2 emissions and an extremely low level of NO.sub.x emissions. Fuel cells therefore hold much promise as a powerful means for curbing the aggravation of the environment by air pollution.
A fuel cell consists of an electrolyte plate sandwiched between two electrodes, the oxygen electrode (cathode) and the fuel electrode (anode). Oxygen or air is supplied to the oxygen electrode whereas hydrogen, a reformed gas obtained by reforming (or processing) hydrocarbons such as natural gas, methanol and petroleum, or coal gas is supplied to the fuel electrode. The supplied oxygen combines electrochemically with the hydrogen in the fuel to produce both electricity and water. Fuel cells can be classified by electrolyte and the four predominant types developed so far comprise alkali electrolyte fuel cells, phosphoric acid electrolyte fuel cells, molten carbonate electrolyte fuel cells, and solid electrolyte fuel cells. Solid electrolyte fuel cells make use of solid oxides and are operated at temperatures of 900.degree.-1,000.degree. C.
In a solid electrolyte fuel cell, oxygen ions (0.sup.2-) migrate through a porous ceramic electrolyte such as yttria stabilized zirconia (ZrO.sub.2 --Y.sub.2 O.sub.3).
The reaction occurring at the oxygen electrode is: EQU 1/20.sub.2 +2e.sup.- =O.sup.2- ( 1).
The reaction taking place at the fuel electrode are: EQU H.sub.2 +O.sup.2- =H.sub.2 O.div.2e.sup.- ( 2) EQU CO+O.sup.2- =CO.sub.2 .div.2e.sup.- ( 3).
The overall reactions in the cell are: EQU 1/20.sub.2 .div.H.sub.2 =H.sub.2 O (4) EQU 1/20.sub.2 .div.CO=CO.sub.2 ( 5).
One major advantage of solid electrolyte fuel cells is that CO can be directly used as a fuel as it would be in the case of molten carbonate electrolyte fuel cells, so that reformed gases obtained by reforming hydrocarbon fuels such as natural gas, methanol and petroleum can be directly supplied to the cell without being passed through a CO converter. Even the fuel reformer (or processor) can be eliminated since the high operating temperature (1,000.degree. C.) combined with the use of Ni as the fuel electrode material enables the fuel gas to be reformed within the cell. This feature offers the added advantage that the fuel supply system for solid electrolyte fuel cells is much simpler in construction than those for other types of fuel cells. Furthermore, the temperature of the gas discharged from solid electrolyte fuel cells is high enough to expand the scope of applications of the heat of the exhaust gases.
The structure of single cells in solid electrolyte fuel cells may be roughly divided into two types. One is a cylindrical type that is described in U.S. Pat. No. 3,460,991, or in the 1983 National Fuel Cell Seminar, Nov. 13-16, 1983, Oakland, Fla., p. 78 and/or in 1985 Fuel Cell Seminar, May 19-22, 1985, Tucson, Ariz., p. 95. The other type is a flat or planar one that is described in U.S. Pat. No. 3,554,808, or U.S. Pat. No. 4,490,445 or 1983 National Fuel Cell Seminar, ibid., p. 74. Solid electrolyte fuel cells which are basically composed of ceramic materials are mechanically brittle and are prone to failure on account of the thermal stress that would be produced by thermal expansion mismatch and nonuniformity in power generation by the cell. These problems have long prevented the development of practically feasible models of solid electrolyte fuel cells. However, as the above-cited U.S. patents show, electrode and separator materials having thermal expansion coefficients substantially the same as those of electrolyte materials were discovered and this led to the fabrication of experimental cells and the conducting of power generation tests on these cells. In particular, the cell system described in 1983 National Fuel Cell Seminar, Nov. 13-16, 1983, Oakland, Fla., p. 78, is designed for circumventing thermal expansion and provides higher reliability for single cells. However, the individual cells in this system are connected by a nickel felt pad which is not capable of complete thermal expansion absorption. In addition, current will flow along the surfaces of thin electrodes and the resulting increase in resistance (ohmic) loss will lead to lower power densities.
Among the planar type cells proposed to date, the one reported in 1983 National Fuel Cell Seminar Nov. 13-16, Oakland, Fla., p. 74, is of a monolithic design constructed by sintering an assembly of an electrolyte, electrodes and a separator and features the ability to produce an extremely high output power density. However, not only does this system require a sophis fabricating technique but it also is substantially incapable of circumventing thermal expansion, thus making it impossible to fabricate a device of large size. In other words, a limited scope of utility is the major defect of this cell design. The fuel cell described in U.S. Pat. No. 4,490,445 comprises single cells which are held between ribbed separator plates and which are each fabricated by sintering an assembly of electrodes (0.003-0.005 inch) coated on opposite sides of a thin disk (0.01-0.02 inch) of ZrO.sub.2 --Y.sub.2 O.sub.3 which serves as an electrolyte. In the absence of a tight gas seal, this cell design is capable of circumventing the thermal expansion of individual cells.
However, in the fuel cell described above, the reactant gases flow diametrically along the principal surfaces of single cells, so the travel path of the reactant gases tends to become longer and the resulting increase in their concentration gradient leads to a greater degree of nonuniformity in the cell output distribution on the cell surfaces.